1. Field of the Invention
The present invention relates to a ZrO2—Al2O3 composite ceramic material having excellent mechanical properties and the capability of preventing low temperature degradation.
2. Disclosure of the Prior Art
In contrast to typical ceramic materials such as alumina, silicon nitride and silicon carbide, yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) containing 2 to 3 mol % of Y2O3 as a stabilizer demonstrate excellent mechanical properties, e.g., high strength and high toughness, and therefore is already widely in practical use. In recent years, the Y-TZP ceramic is beginning to be applied as a biomaterial for artificial joints, artificial tooth roots, abutment, crown and so on.
However, the Y-TZP ceramic has a problem that a phase transformation from metastable tetragonal ZrO2 to monoclinic ZrO2 proceeds at a relatively low temperature region, e.g., 200 to 300° C., while accompanying a volume expansion of about 4.6%. Due to microcracks developed in the Y-TZP ceramic by this volume expansion, a considerable deterioration in mechanical properties occurs. In addition, it is well known that the phase transformation is further accelerated under a wet condition (e.g., in vivo environments). As one of major causes for such a low temperature degradation of the Y-TZP ceramic, it is believed that trivalent yttrium ions are interstitially dissolved in eight-coordinate positions of tetravalent zirconium ions, so that oxygen defects are generated in ZrO2 lattice by the difference in valence level.
On the other hand, ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP) containing CeO2 as the stabilizer are also widely known. In this case, since tetravalent cerium ions are dissolved into the ZrO2 lattice, the oxygen defects are not generated. It has been supported by lots of experimental data that the low temperature degradation of the Ce-TZP ceramic does not happen crystallographically. In addition, this ceramic demonstrates a remarkably high toughness. However, there is a problem that the mechanical strength and hardness of the Ce-TZP ceramic are much lower than those of the Y-TZP ceramic. Consequently, it has been a significant barrier to the practical use.
For example, as disclosed in Japanese Patent Early Publication No. 63-156063 or No. 63-123861, both of ceria and yttria is used as the stabilizer to obtain a high-strength ZrO2 sintered body. This ZrO2 sintered body includes a partially stabilized zirconia mainly composed of tetragonal ZrO2 or tetragonal ZrO2 and cubic ZrO2, which contains 4 to 6 mol % of ceria (CeO2) and 2 to 6 mol % of yttria (YO1.5) as the stabilizer, and a second phase of at least one selected from alumina, spinel and mullite. In this case, it is believed that thermal stability is improved because ZrO2 of this sintered body has a structure closer to a cubic crystal that is the high-temperature stable phase of ZrO2, as compared with the tetragonal ZrO2 containing only yttria as the stabilizer.
However, there is another problem that, during sintering, crystal grains of the cubic ZrO2 easily become larger in size than the case of the tetragonal ZrO2. That is, abnormal grain growth of ZrO2 easily occurs. Therefore, it is difficult to ensure sufficient strength, hardness and wear resistance with reliability. In addition, the presence of the second phase such as alumina and mullite makes difficult to complete the sintering of ZrO2 matrix containing yttria as the stabilizer. As a result, a specialized sintering technique such as pressure sintering or HIP will be needed to obtain a dense sintered body. This leads to an increase in production cost and restricts the production of a sintered body having a complex shape.